Many techniques that detect nucleobases to which substances such as biotin are artificially introduced by immunoassays using antibodies in nucleic acid (e.g., DNA, RNA) detection have been reported. Techniques that detect naturally occurring modified nucleobases (e.g., methylcytosine, hydroxymethylcytosine) by immunoassays are also reported (Patent Literature 1, and Non Patent Literature 1 and 2).